MSGOP(2) System Calls Manual MSGOP(2)
NAME
msgrcv, msgsnd - System V message queue operations
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/msg.h>
int msgsnd(int msqid, const void msgp[.msgsz], size_t msgsz,
int msgflg);
ssize_t msgrcv(int msqid, void msgp[.msgsz], size_t msgsz, long msgtyp,
int msgflg);
DESCRIPTION
The msgsnd() and msgrcv() system calls are used to send messages to,
and receive messages from, a System V message queue. The calling
process must have write permission on the message queue in order to
send a message, and read permission to receive a message.
The msgp argument is a pointer to a caller-defined structure of the
following general form:
struct msgbuf {
long mtype; /* message type, must be > 0 */
char mtext[1]; /* message data */
};
The mtext field is an array (or other structure) whose size is speci-
fied by msgsz, a nonnegative integer value. Messages of zero length
(i.e., no mtext field) are permitted. The mtype field must have a
strictly positive integer value. This value can be used by the receiv-
ing process for message selection (see the description of msgrcv() be-
low).
msgsnd()
The msgsnd() system call appends a copy of the message pointed to by
msgp to the message queue whose identifier is specified by msqid.
If sufficient space is available in the queue, msgsnd() succeeds imme-
diately. The queue capacity is governed by the msg_qbytes field in the
associated data structure for the message queue. During queue creation
this field is initialized to MSGMNB bytes, but this limit can be modi-
fied using msgctl(2). A message queue is considered to be full if ei-
ther of the following conditions is true:
• Adding a new message to the queue would cause the total number of
bytes in the queue to exceed the queue's maximum size (the
msg_qbytes field).
• Adding another message to the queue would cause the total number of
messages in the queue to exceed the queue's maximum size (the
msg_qbytes field). This check is necessary to prevent an unlimited
number of zero-length messages being placed on the queue. Although
such messages contain no data, they nevertheless consume (locked)
kernel memory.
If insufficient space is available in the queue, then the default be-
havior of msgsnd() is to block until space becomes available. If
IPC_NOWAIT is specified in msgflg, then the call instead fails with the
error EAGAIN.
A blocked msgsnd() call may also fail if:
• the queue is removed, in which case the system call fails with errno
set to EIDRM; or
• a signal is caught, in which case the system call fails with errno
set to EINTR;see signal(7). (msgsnd() is never automatically
restarted after being interrupted by a signal handler, regardless of
the setting of the SA_RESTART flag when establishing a signal han-
dler.)
Upon successful completion the message queue data structure is updated
as follows:
• msg_lspid is set to the process ID of the calling process.
• msg_qnum is incremented by 1.
• msg_stime is set to the current time.
msgrcv()
The msgrcv() system call removes a message from the queue specified by
msqid and places it in the buffer pointed to by msgp.
The argument msgsz specifies the maximum size in bytes for the member
mtext of the structure pointed to by the msgp argument. If the message
text has length greater than msgsz, then the behavior depends on
whether MSG_NOERROR is specified in msgflg. If MSG_NOERROR is speci-
fied, then the message text will be truncated (and the truncated part
will be lost); if MSG_NOERROR is not specified, then the message isn't
removed from the queue and the system call fails returning -1 with er-
rno set to E2BIG.
Unless MSG_COPY is specified in msgflg (see below), the msgtyp argument
specifies the type of message requested, as follows:
• If msgtyp is 0, then the first message in the queue is read.
• If msgtyp is greater than 0, then the first message in the queue of
type msgtyp is read, unless MSG_EXCEPT was specified in msgflg, in
which case the first message in the queue of type not equal to msg-
typ will be read.
• If msgtyp is less than 0, then the first message in the queue with
the lowest type less than or equal to the absolute value of msgtyp
will be read.
The msgflg argument is a bit mask constructed by ORing together zero or
more of the following flags:
IPC_NOWAIT
Return immediately if no message of the requested type is in the
queue. The system call fails with errno set to ENOMSG.
MSG_COPY (since Linux 3.8)
Nondestructively fetch a copy of the message at the ordinal po-
sition in the queue specified by msgtyp (messages are considered
to be numbered starting at 0).
This flag must be specified in conjunction with IPC_NOWAIT, with
the result that, if there is no message available at the given
position, the call fails immediately with the error ENOMSG. Be-
cause they alter the meaning of msgtyp in orthogonal ways,
MSG_COPY and MSG_EXCEPT may not both be specified in msgflg.
The MSG_COPY flag was added for the implementation of the kernel
checkpoint-restore facility and is available only if the kernel
was built with the CONFIG_CHECKPOINT_RESTORE option.
MSG_EXCEPT
Used with msgtyp greater than 0 to read the first message in the
queue with message type that differs from msgtyp.
MSG_NOERROR
To truncate the message text if longer than msgsz bytes.
If no message of the requested type is available and IPC_NOWAIT isn't
specified in msgflg, the calling process is blocked until one of the
following conditions occurs:
• A message of the desired type is placed in the queue.
• The message queue is removed from the system. In this case, the
system call fails with errno set to EIDRM.
• The calling process catches a signal. In this case, the system call
fails with errno set to EINTR. (msgrcv() is never automatically
restarted after being interrupted by a signal handler, regardless of
the setting of the SA_RESTART flag when establishing a signal han-
dler.)
Upon successful completion the message queue data structure is updated
as follows:
msg_lrpid is set to the process ID of the calling process.
msg_qnum is decremented by 1.
msg_rtime is set to the current time.
RETURN VALUE
On success, msgsnd() returns 0 and msgrcv() returns the number of bytes
actually copied into the mtext array. On failure, both functions re-
turn -1, and set errno to indicate the error.
ERRORS
msgsnd() can fail with the following errors:
EACCES The calling process does not have write permission on the mes-
sage queue, and does not have the CAP_IPC_OWNER capability in
the user namespace that governs its IPC namespace.
EAGAIN The message can't be sent due to the msg_qbytes limit for the
queue and IPC_NOWAIT was specified in msgflg.
EFAULT The address pointed to by msgp isn't accessible.
EIDRM The message queue was removed.
EINTR Sleeping on a full message queue condition, the process caught a
signal.
EINVAL Invalid msqid value, or nonpositive mtype value, or invalid ms-
gsz value (less than 0 or greater than the system value MSGMAX).
ENOMEM The system does not have enough memory to make a copy of the
message pointed to by msgp.
msgrcv() can fail with the following errors:
E2BIG The message text length is greater than msgsz and MSG_NOERROR
isn't specified in msgflg.
EACCES The calling process does not have read permission on the message
queue, and does not have the CAP_IPC_OWNER capability in the
user namespace that governs its IPC namespace.
EFAULT The address pointed to by msgp isn't accessible.
EIDRM While the process was sleeping to receive a message, the message
queue was removed.
EINTR While the process was sleeping to receive a message, the process
caught a signal; see signal(7).
EINVAL msqid was invalid, or msgsz was less than 0.
EINVAL (since Linux 3.14)
msgflg specified MSG_COPY, but not IPC_NOWAIT.
EINVAL (since Linux 3.14)
msgflg specified both MSG_COPY and MSG_EXCEPT.
ENOMSG IPC_NOWAIT was specified in msgflg and no message of the re-
quested type existed on the message queue.
ENOMSG IPC_NOWAIT and MSG_COPY were specified in msgflg and the queue
contains less than msgtyp messages.
ENOSYS (since Linux 3.8)
Both MSG_COPY and IPC_NOWAIT were specified in msgflg, and this
kernel was configured without CONFIG_CHECKPOINT_RESTORE.
STANDARDS
POSIX.1-2008.
The MSG_EXCEPT and MSG_COPY flags are Linux-specific; their definitions
can be obtained by defining the _GNU_SOURCE feature test macro.
HISTORY
POSIX.1-2001, SVr4.
The msgp argument is declared as struct msgbuf * in glibc 2.0 and 2.1.
It is declared as void * in glibc 2.2 and later, as required by SUSv2
and SUSv3.
NOTES
The following limits on message queue resources affect the msgsnd()
call:
MSGMAX Maximum size of a message text, in bytes (default value: 8192
bytes). On Linux, this limit can be read and modified via
/proc/sys/kernel/msgmax.
MSGMNB Maximum number of bytes that can be held in a message queue (de-
fault value: 16384 bytes). On Linux, this limit can be read and
modified via /proc/sys/kernel/msgmnb. A privileged process
(Linux: a process with the CAP_SYS_RESOURCE capability) can in-
crease the size of a message queue beyond MSGMNB using the ms-
gctl(2) IPC_SET operation.
The implementation has no intrinsic system-wide limits on the number of
message headers (MSGTQL) and the number of bytes in the message pool
(MSGPOOL).
BUGS
In Linux 3.13 and earlier, if msgrcv() was called with the MSG_COPY
flag, but without IPC_NOWAIT, and the message queue contained less than
msgtyp messages, then the call would block until the next message is
written to the queue. At that point, the call would return a copy of
the message, regardless of whether that message was at the ordinal po-
sition msgtyp. This bug is fixed in Linux 3.14.
Specifying both MSG_COPY and MSC_EXCEPT in msgflg is a logical error
(since these flags impose different interpretations on msgtyp). In
Linux 3.13 and earlier, this error was not diagnosed by msgrcv(). This
bug is fixed in Linux 3.14.
EXAMPLES
The program below demonstrates the use of msgsnd() and msgrcv().
The example program is first run with the -s option to send a message
and then run again with the -r option to receive a message.
The following shell session shows a sample run of the program:
$ ./a.out -s
sent: a message at Wed Mar 4 16:25:45 2015
$ ./a.out -r
message received: a message at Wed Mar 4 16:25:45 2015
Program source
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <time.h>
#include <unistd.h>
struct msgbuf {
long mtype;
char mtext[80];
};
static void
usage(char *prog_name, char *msg)
{
if (msg != NULL)
fputs(msg, stderr);
fprintf(stderr, "Usage: %s [options]\n", prog_name);
fprintf(stderr, "Options are:\n");
fprintf(stderr, "-s send message using msgsnd()\n");
fprintf(stderr, "-r read message using msgrcv()\n");
fprintf(stderr, "-t message type (default is 1)\n");
fprintf(stderr, "-k message queue key (default is 1234)\n");
exit(EXIT_FAILURE);
}
static void
send_msg(int qid, int msgtype)
{
time_t t;
struct msgbuf msg;
msg.mtype = msgtype;
time(&t);
snprintf(msg.mtext, sizeof(msg.mtext), "a message at %s",
ctime(&t));
if (msgsnd(qid, &msg, sizeof(msg.mtext),
IPC_NOWAIT) == -1)
{
perror("msgsnd error");
exit(EXIT_FAILURE);
}
printf("sent: %s\n", msg.mtext);
}
static void
get_msg(int qid, int msgtype)
{
struct msgbuf msg;
if (msgrcv(qid, &msg, sizeof(msg.mtext), msgtype,
MSG_NOERROR | IPC_NOWAIT) == -1) {
if (errno != ENOMSG) {
perror("msgrcv");
exit(EXIT_FAILURE);
}
printf("No message available for msgrcv()\n");
} else {
printf("message received: %s\n", msg.mtext);
}
}
int
main(int argc, char *argv[])
{
int qid, opt;
int mode = 0; /* 1 = send, 2 = receive */
int msgtype = 1;
int msgkey = 1234;
while ((opt = getopt(argc, argv, "srt:k:")) != -1) {
switch (opt) {
case 's':
mode = 1;
break;
case 'r':
mode = 2;
break;
case 't':
msgtype = atoi(optarg);
if (msgtype <= 0)
usage(argv[0], "-t option must be greater than 0\n");
break;
case 'k':
msgkey = atoi(optarg);
break;
default:
usage(argv[0], "Unrecognized option\n");
}
}
if (mode == 0)
usage(argv[0], "must use either -s or -r option\n");
qid = msgget(msgkey, IPC_CREAT | 0666);
if (qid == -1) {
perror("msgget");
exit(EXIT_FAILURE);
}
if (mode == 2)
get_msg(qid, msgtype);
else
send_msg(qid, msgtype);
exit(EXIT_SUCCESS);
}
SEE ALSO
msgctl(2), msgget(2), capabilities(7), mq_overview(7), sysvipc(7)
Linux man-pages 6.7 2023-10-31 MSGOP(2)
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